This invention relates in general to multi-phase inverter systems for AC generators which incorporate static electrical power conversion elements, and more particularly, to multi-phase inverters which provide an AC output voltage which is controllable, independent of a DC input voltage, over a predetermined range.
The invention is particularly applicable to aviation electrical power generation requirements which rely on state of the art electronic features, such as variable speed constant frequency (VSCF) electrical power generators with DC-link control, which are needed aboard present and future military aircraft and for multi-engine commercial aircraft as well. It will be appreciated that the invention has broader applications such as wherever it is needed to provide output power voltage regulation from a static inverter without mandating the controlling of the DC input voltage to the inverter.
Each VSCF generator unit aboard an aircraft includes a multi-phase or three-phase power inverter paired with a propulsion engine in order that each engine can be used to help fulfill the requirement of system redundancy to enhance operational safety. This ensures that the entire electrical power output needs for the aircraft will be met in order to keep the aircraft operational. The VSCF system according to U.S Pat. No. 4,554,501, entitled "Variable Speed Constant Frequency Power System With Auxiliary DC Output" is assigned to the same assignee as the present invention, and it is hereby incorporated by reference into the present application. Another variable speed generator is of the type described in U.S. Pat. No. 4,851,723 entitled "Coolant Pump System for Variable Speed Generators" which also is assigned to the present assignee, and since it also provides a proper environment for the utilization of the presently intended multi-phase inverter system it is likewise incorporated by reference as the second incorporated U.S. Patent.
The three-phase inverters that are used with DC-link frequency conversion for aircraft power generation systems, operating at 400 Hertz and 20 to 50 Kilovolt-amps (KVA), are a subset of static inverters employing semiconductor switching devices which are used for many other applications. These include industrial drive, power conditioning, and the generation of controllable leading or lagging reactive current. In each of these applications, there is a common problem which is difficult to solve. It is the elimination and/or reduction to filterable minimum values of troublesome AC harmonic frequencies which are whole number multiples of the fundamental frequency in hertz. One approach to solving the problem of harmonic neutralization is according to U.S. Pat. No. 4,870,557, entitled "Simplified Quasi-Harmonic Neutralized High Power Inverters" which has been assigned to the same assignee as the present invention. The above assigned patent is hereby incorporated by reference into the present application and will be hereinafter referenced as the third incorporated U.S. patent. This incorporated U.S. patent takes advantage of the double fed 24-pulse inverter comprised of standard groups of inverters such as four six-pulse inverters connected to reduce the complexity of and to half the number of normally required phase shifting transformers with open primary winding neutrals. Furthermore, the secondary winding of at least one of the phase shifting transformers is a delta or a wye winding connected to a neutral line in order to simplify the quasi-harmonic neutralization of the output.
Another timely approach directed towards solving the problem of harmonic neutralization is with the implementation of 12-pulse and 24-pulse inverters assembled from pairs of six-pulse inverters with strategic placement of time-paired notching sets for the reversal of the pole voltage. This approach places each notch with a timing duration within a limited band near to the leading and trailing portions of a half-wave periodicity, and it significantly decreases the original phase displacement of the pole voltages, while offering the simplicity of a single output coupling transformer. This is the subject of pending U.S. Ser. No. 07/612,647, filed Nov. 13, 1990, entitled "Static Inverter", assigned to the same assignee as the present invention, and it is hereby incorporated by reference into the present application and will be hereinafter referenced as the first incorporated U.S. application for patent. The clear benefit to be derived in notching the switching output of the inverters, within this leading and trailing portion timing range, is the affect on some of the most significant harmonics. This viewpoint is for some of the practical applications of the quasi-harmonic neutralized output voltage waveform in which some harmonics are reduced and/or eliminated, i.e. the 5th and 7th harmonics are eliminated and the 11th and 13th are significantly reduced.
One of the principal practical considerations for the realization of static inverter harmonic neutralization of the type described is that the AC output voltage is harmonically neutralized over a predetermined range for which voltage control of the fundamental frequency component is independent of the DC source voltage. Another problem related to the above concerns is that the controlled notching of the multi-pole inverter circuit should reduce the harmonic content of the inverter circuit to a filterable minimum of harmonic voltage components immediately next adjacent to twelve times the fundamental frequency.
The solution should further concern the provision that all remaining harmonic voltage components immediately adjacent to odd number multiples of six times the fundamental frequency are to be cancelled, while a substantial decrease in the 11th and 13th harmonics is accomplished over the entire range of control.